Menu

Mermaid’s purses (eggcases)

As we’re learning oh so well, Chondrichthyes are quite the unique class of fish. These sharks, skates and rays are equipped with skeletons made of boneless, flexible cartilage, true upper AND lower jaws, electric-field sensory organs (Ampullae of Lorenini).. the list goes on. The males even have clingy claspers under their bellies (more on those later). But one popular trait specific to some elasmobranch species is the rigid eggcase, colloquially the mermaid’s purse or devil’s purse. Check out the photos for more details about these strange shark makers!

Three eggcases of Big Skate embryos (Beringraja binoculata). The left-most one hosts four live skates! They are about three months old now.

Take a closer look at those little swimmers! Big Skate eggcases are one of only two species known to regularly carry more than one embryo. The other is Beringraja pulchra in the western Pacific.Specialists at the BMay Aquarium installed plastic windows on ventral sides of the eggcases then displayed them upside down to give visitors a peak inside the organic incubators.

The eggcases of B. binoculata are one of the largest of eastern Pacific skates! You can really see their concave structure, arched dorsal surface, flat ventral (under) side and blunt, broad horns.

Another Big Skate species mermaids purse at the MBay Aquarium. Notice the parallel lateral keels along the back side of the eggcase. These rigid cases can incubate embryos anywhere between four to 16 months! Incubation rates are dependent on water temperature, resulting in about a six-month duration while in captivity.

A Swell Shark (Cephaloscyllium ventriosum) incubates at the Monterey Bay Aquarium. C. ventriosum’s large purse-like eggcases are smooth and a bit thick with tapering horns and long tendrils for hooking onto substrate. Its name comes from its ability to distend its stomach and swell its trunk to a near-spherical shape by swallowing water or air.

Eggcases aren’t like anything else in the sea. They are leathery, rounded-rectangular-ish, dark colored and stiff — All traits to better hide them along the seabed from predators! Think of them as birds eggs: they are fertilized before they are laid; their shells are protein-based and collagen-like and also have hooks to better latch onto substrate or natural stone seafloor so they don’t get carried far away from where their mothers dropped them off.

Some live-bearing sharks may keep eggcases inside themselves rather than laying them on the sea floor, but these eggs aren’t always rigid like mermaids purses. Rather, they can be soft and squishy like water balloons! The embryo sharks absorb these yolk sacs until they are ready to be born ready-to-swim into the ocean blue.